Fluorescent lighting provides an energy-efficient alternative to common incandescent lighting. However, the adoption of fluorescent lighting has been hindered in certain applications. The quality of object color under fluorescent lighting is an important aspect of the value of the light source. There are certain applications where good color rendition is very important. Such applications are illumination in commercial units, where the true color perception of products are desired, such as clothing stores, fresh food stores, and photographic and cinema studios.
In general, the quality of object color has been described in terms of color rendering, which is a measure of the degree to which the psycho-physical colors of objects illuminated by a light source conform to those of a reference illuminant for specified conditions. Color rendering as used here refers to the accurate representation of object colors compared to those same objects under a reference source. Some light sources, for example, fluorescent lighting, have been known to have poor color rendering, such that they can produce visible light having muted light in the red region of the spectrum and an overemphasized amount of light in the yellow region. Such lighting has been observed to produce cold or sterile light and light that generally washes out colors of the items being illuminated.
Light sources have also been described in terms of their apparent color, expressed as color temperature, which is the temperature of a black body that emits radiation of about the same chromaticity as the radiation considered. A light source having a color temperature of 3000 kelvin has a larger red component than a light source having a color temperature of 4100 kelvin. White light typically has a color temperature (more precisely, a correlated color temperature, CCT) of between about 4000 and 8000 K. White light with a color temperature of 4000 has a yellowish color. White light with a CCT of 8000 K is more bluish in color, and may be referred to as “cool white”. There is some evidence to suggest that light having a substantial blue component has an effect upon the human body in terms of sleep cycles and hormonal effects. It is generally known, in particular, that very blue light can be effective in adjusting the human circadian rhythm.
However, lamps having a high color temperature lamps (and thus having a substantial blue component), are frequently considered to be objectionable in terms of their color rendering of objects, i.e., their appearance. What is desired, then, is a light source having a spectrum with improved color rendering even at high or very high color temperatures.